Exhaust gas purification apparatus, exhaust gas purification method, and sulfur component trapping agent for internal combustion engine

ABSTRACT

An object of the present invention is to provide a new exhaust gas purification apparatus for an internal combustion engine operated under a condition of an air fuel ratio leaner than a theoretical air fuel ratio, a method for purification of exhaust gas and an exhaust gas purification catalyst, which is suitable for suppressing degradation of the NOx purification catalyst by sulfur components.  
     An exhaust gas purification apparatus for an internal combustion engine, which comprises an exhaust gas passage for an internal combustion engine into which exhaust gas of lean air fuel ratio and rich or stoichiometric air fuel ratio flows, a NOx trapping catalyst that functions to trap NOx in the exhaust gas when the air fuel ratio is lean, a sulfur component trapping agent for trapping sulfur components in the exhaust gas, which is disposed before the NOx trapping catalyst, and a catalyst for oxidizing the sulfur components, which is disposed before the sulfur component trapping agent, wherein the sulfur component trapping agent has a trapping rate of 85 % or more of an amount of inflow sulfur in a trapping test at a flow rate of 150 ppm SO 3 -5% O 2 —balance being N 2  gas per 1.5 moles of the sulfur trapping agent at 300° C. and a space velocity of 30,000/h for 1 hour; and the sulfur component trapping agent has a release rate of sulfur amount of 5 % or less of sulfur trapped in the sulfur component trapping agent in a release test under a flow of a 3000 ppm H 2 -600 ppm C3H6-3000 ppm O 2 -3.5 % CO—balance being N 2  gas at a temperature elevation rate of 10° C. /min from 250 to 750° C. at an sulfur component trapping agent entrance, after the trapping test.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification apparatusand exhaust gas purification method for an internal combustion engineoperated under a lean burn condition, wherein fuel is leaner than atheoretical air/fuel ratio.

BACKGROUND ART

In recent years, there have been attention to lean burn engines for leanfuel combustion in terms of an air fuel ratio. The air fuel ratiorepresents a ratio of air to fuel in gas.

Three way catalysts that have been used for exhaust gas purification ofengines for theoretical air fuel ratio (stoichiometry) combustion havedifficulty in purifying NOx. Thus, exhaust gas purifying catalysts forlean burn engines are investigated. One of them is disclosed in Japanesepatent laid-open 11-319564(Patent publication 1) is disclosed. Thepublication discloses one of NOx absorbing metal oxides selected fromalkali metals, alkaline earth metals and rare earth metals and a noblemetal supported on a porous support. According to the publication, NOxis effectively purified in a lean state of the air fuel ratio by usingthe catalysts. However, it has been known that since the exhaust gasfrom the lean burn engines contains SOx derived from sulfur contained ingasoline, activity of the catalysts is deteriorated by SOx, which reactswith components in the NOx trapping catalysts.M-O+SO₂+O₂→M-SO₄  (1)

(M: alkali metals or alkaline earth metals)

When the catalytic activity by sulfur component happens, it has beencontemplated that the catalysts are exposed to gas of high temperatureand rich or stoichiometric s air fuel ratio so as to carry out areaction shown by the formula (2), thereby to regenerate the catalyst bydesorbing the sulfur component (S purge).M-SO₄→M-O+SO₂+O₂  (2)

(M: alkali metals or alkaline earth metals)

In addition to the S purge, a method for controlling absorption amountof SOx is disclosed in Japanese patent laid-open 8-192051 (Patentdocument 2) wherein composite oxides of Ti and Zr are used as a supportwhereby the activity of the NOx trapping catalysts is maintained even ifsulfur components are present in the exhaust gas.

Further, Japanese patent laid-open 11-169708 (Patent document 3)discloses a SOx trapping agent is disposed before the NOx trappingcatalyst, thereby to trap SOx at the time of lean air fuel ratio andrelease SOx at the time of rich air fuel ratio, whereby an amount ofinflow of sulfur components is reduced to suppress the sulfur poisoningto the NOx trapping catalysts or NOx purification catalyst.

DESCRIPTION OF INVENTION

The S purge makes fuel cost worse because it employs rich air fuelratio. When a large amount of sulfur components adheres to the NOxtrapping catalyst or NOx purification catalyst, an amount of a reducingagent for removing sulfur components as the amount of adhered sulfurcomponents increases. As a result, a degree of rich air fuel ratiobecomes large (the air fuel ratio becomes small), which makes the fuelcost worse. Further, once a large amount of sulfur components adhere tothe NOx trapping catalyst, sulfur components that strongly react withthe NOx trapping agent are hard to be removed thereby to lower thepurification performance.

On the other hand, at the time of regeneration of NOx catalysts, sulfurcomponents react with hydrogen, etc contained in the exhaust gas andexhausted as hydrogen sulfide (H₂S) as shown in the formula 3, whichmakes the exhaust gas smell.M-SO₄+H₂→M-O+H₂S+O₂  (3)

According to the technologies disclosed in Japanese patent laid-open8-192051 (Patent document 2), adhesion of sulfur components to the NOxtrapping catalyst at the time of lean air fuel ratio is suppressed andNOx trapping performance becomes lower.

According to the technologies disclosed in Japanese patent laid-open11-169708 (Patent document 3), though an amount of inflow of sulfurcomponents at the time of lean air fuel ratio, the sulfur components arereleased from the SOx trapping agent disposed in front of the NOxtrapping catalyst at the time of rich air fuel ratio, and the NOxcatalyst is sulfur poisoned by the released sulfur. Accordingly, Spurgeis necessary, which leads to the above problem, however.

Further, regeneration of sulfur trapping agent is necessary at the timeof rich air fuel ratio where sulfur components are released, sulfurcomponents are discharged though a NOx trapping rate of NOx trappingcatalyst increases.

In addition to the above, since development for increasing a NOxpurification rate under lean burn conditions has been made, an exhaustgas purification systems that does not discharge sulfur components hasnever been investigated. Since regeneration of NOx trapping catalyst andsulfur trapping agent release sulfur components, these sulfur componentsshould be controlled.

An object of the present invention is to provide a sulfur componenttrapping agent that removes the above-mentioned disadvantages and cantrap sulfur components for a long time, an exhaust gas purificationapparatus and an exhaust gas purification method.

The present invention relates to an exhaust gas purification apparatusfor an internal combustion engine capable of lean burn operation at anair fuel ratio leaner than the theoretical air fuel ratio (18 or more),which comprises an exhaust gas passage of an internal combustion engineinto which exhaust gas of an air fuel ratio of 14.7 or less flows, anNOx purification catalyst for trapping NOx in the exhaust gas at thetime of lean air fuel ratio, and a sulfur component trapping agent,disposed in a previous stage of or before the NOx tapping catalyst, forcapable of trapping sulfur components in the exhaust gas, wherein thesulfur trapping agent does not desorb (or does not substantiallyrelease) at the time of rich or stoichiometric air fuel ratio. Thesulfur components include sulfur and sulfur compounds, which are derivedfrom gasoline, light oil, lubricants, etc, and are present in theexhaust gas.

The words “does not desorb” are used to mean that a damaging amount ofsulfur components are not released. For example, the sulfur componenttrapping agent exhibit a trapping rate of 85% or more with respect to aninflow amount of sulfur components when gas composed of 150 ppm SO₂-5%O₂—balance being N₂ is flown through the sulfur component trapping agentat an SV of 30,000/h and at 300° C. per 1.5 mol of the sulfur componenttrapping agent, and after the above test, the sulfur trapping agentexhibits an amount of 5% or less of sulfur components released from thesulfur component trapping agent per an amount of trapped sulfurcomponents under conditions that a gas composition is 2000 ppmH₂-500ppmC₃H₆-3000 ppmO₂-3.5% CO—balance being N₂, which is flown through thesulfur components trapping and temperatures are elevated from 300 to750° C. at a rate of 10° C. /min.

Another feature of the present invention resides in providing an exhaustgas purification apparatus that installs a sulfur component trappingagent capable of sufficiently trapping sulfur components under rich orstoichiometric conditions.

A still another feature of the present invention resides in that asulfur component trapping agent is disposed in the previous stage of theNOx trapping catalyst so as to suppress reduction of catalytic activityby the sulfur adhesion and a catalyst for oxidizing sulfur in theexhaust gas is disposed in the previous stage of the sulfur componenttrapping agent. The sulfur trapping agent includes a support that holdssulfur components in the exhaust gas.

As is disclosed in Japanese laid-open 11-169708, bonding power of SO₂ tothe sulfur component trapping agent is weaker than that of SO₃, and thesulfur components are hard to be trapped. This is because the number ofoxygen atoms in SO₃ is large and because there is imbalance ofelectrons. Thus, acidity of the SO₃ is high. Accordingly, the sulfurcomponents in the exhaust gas are more easily trapped in the form of SO₃than SO₂, as shown in the formula 4.SO₂+O₂→SO₃  (4)

Further, when SO₂ gas enters the sulfur trapping agent, there is apossibility of reaction represented by the formula (5) in addition tothe formula (2) thereby to produce sulfites as well as sulfates.M-O+SO₂→M-SO₃  (5)

(M: metal atom for the sulfur trapping agent)

In general, sulfites are less stable than sulfates, and thusdecomposition temperature of the sulfites is low. Accordingly, ifsulfites are produced dominantly, a temperature of the exhaust gaselevates, and the temperature of the sulfur trapping agent elevates,decomposition reaction of the sulfites takes place thereby to releasesulfur components from the sulfur trapping agent and to cause a problemthat a NOx trapping catalyst disposed at a post stage may be poisoned bysulfur components.

By disposing a catalyst for oxidizing sulfur components in the exhaustgas in front of the sulfur component trapping agent, sulfur componentsare trapped as SO₃. As examples of sulfur component oxidizing catalysts,noble metals such as Rh, Pt, Pd, etc, and any other catalysts that canoxidize sulfur can be employed.

Although a sulfur component trapping agent supported together withsulfur component oxidizing component are supported on the same supportmay trap the sulfur components, such sulfur trapping agents are notproper if the sulfur oxidizing catalysts oxidize sulfur components andaccelerate decomposition, which leads to release of sulfur from thesulfur component trapping agent.

The sulfur component trapping agent according to the present inventionis featured by that the sulfur trapping agent is an oxide or carbonateof the metals and is capable of forming sulfates or sulfites that hardlyrelease or discharge trapped sulfur components even when a temperatureof the sulfur trapping agent elevates. The sulfur trapping agent isfeatured by sulfates of alkali metals such as Li, Na, K, Rb, Cs, etc,alkaline earth metals such as Mg, Ca, Sr, Ba, etc, or Ce, Al, Y, La, Ni,etc, which form sulfates having high melting points or highdecomposition temperatures, and is also featured by not substantiallycontaining such components as Ph, Pt, Pd, etc.

If the components such as noble metals that effect reaction ofdecomposition of sulfites are contained in the SOx trapping agent, thesulfur components trapped as the sulfites in the SOx trapping agent maybe released in accordance with the formula (2) in a rich air fuel ratio.Accordingly, it is preferable that any components that contribute todecomposition reaction of sulfites are not contained in the sulfurcomponent trapping agent. A preferable total amount of the noble metalsshould be 0.4% by weight or less, and more preferably, 0.3% by weight orless.

The present invention also relates to a filter, disposed in an exhaustgas passage of an internal combustion engine, for purification ofexhaust gas from an internal combustion engine, a part of which carriesa sulfur oxidizing catalyst and the other part of which carries a sulfurcomponent trapping agent. The present invention relates to an exhaustgas purification apparatus using the exhaust gas purification filter.The exhaust gas purification apparatus is featured by disposing thefilter in such a manner that the exhaust gas is introduced into thesulfur component oxidizing catalyst side and discharged from the sulfurcomponent trapping agent side.

The sulfur component oxidizing catalyst may be supported on the surfaceof the upstream side of the filter, which is a plate or shaped flatfilter, as same as in diesel particulate filters.

The sulfur component trapping agent or the exhaust gas purificationapparatus according to the present invention can suppress degradation ofthe NOx purification catalyst for the internal combustion engine, whichis operated under the lean burn condition of which the air fuel ratio isleaner than that of theoretical air fuel ratio.

Further, the present invention can provide an internal combustion enginefor automobiles and an exhaust gas purification apparatus that has goodfuel consumption, suppressed degradation of a NOx purification catalystcaused by sulfur component and a small amount of sulfur componentemission.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an arrangement of a NOx trapping catalyst, sulfur componenttrapping agent and sulfur component oxidizing catalyst.

FIG. 2 is a graph showing a SOx trapping rate with respect todisposition and non-disposition of the sulfur oxidizing catalyst.

FIG. 3 is a graph showing release of H₂S from sodium sulfate.

FIG. 4 is a graph showing release of SO₂ LiPt/Al₂O₃.

FIG. 5 is a graph showing a sulfur release rate with respect to acontent of Rh+Pt+Pd.

FIG. 6 is a diagrammatic view of a DPF wherein the sulfur componentoxidizing catalyst is supported on the upper-stream side of DPF and thesulfur component trapping agent is supported on the down-stream side ofthe DPF.

FIG. 7 shows an arrangement of the sulfur component trapping agent,which is disposed immediately after the engine.

FIG. 8 is a graph showing degradation of NOx trapping catalyst 1depending on time.

FIG. 9 is a graph showing relationship between a sulfur trapping rate bythe sulfur component trapping agent and sulfur purge frequency.

FIG. 10 is a graph showing an amount of sulfur component on the NOxtrapping catalyst depending on gas atmosphere.

FIG. 11 is a graph showing regeneration of the sulfur component trappingagent by sulfur purging.

FIG. 12 is a diagrammatic view of the exhaust gas purification apparatusaccording to the present invention.

BEST EMBODIMENTS FOR PRACTICING THE INVENTION

(Investigations of Sulfur Component Trapping Agents)

As the sulfur component trapping agents, components that hardly releaseand discharge sulfur components even when a temperature of exhaust gaschanges, which leads to temperature change of the sulfur componenttrapping agent were investigated.

Since the temperature of the exhaust gas varies from room temperature to650° C., the sulfates of the sulfur component trapping agents shouldpreferably have a melting temperature or decomposition temperature of750° C. or or higher. When the melting temperature or decompositiontemperature of the sulfates is 750° C., decomposition of sulfates hardlyoccurs and release and discharge of sulfur components are prevented.Table 1 shows melting points or decomposition temperatures of varioussulfates (Refer to Chemistry Encyclopedia: Basic Version II, JapanChemical Society, Maruzen Publishing Co.). TABLE 1 Melting points anddecomposition temperatures Melting point Element for a sulfur (*;decomposition trapping agent Sulfate temperature) Li Li₂SO₄ 1256 NaNa₂SO₄  884 K K₂SO₄ 1069 Rb Rb₂SO₄ 1060 Cs Cs₂SO₄ 1010 Mg MgSO₄ 1185 CaCaSO₄ 1450 Sr SrSO₄ 1605 Ba BaSO₄ 1580 Mn MnSO₄  700 Ce Ce₂(SO₄)₃  920*Al Al₂(SO₄)₃  770 La La₂(SO₄)₃  1150* Fe Fe₂(SO₄)₃  480* Y Y₂(SO₄)₃ 1000* Zn ZnSO₄  600* Co CoSO₄  735 Zr Zr(SO₄)₂•4H₂O  380* Cu CuSO₄  200Ni Ni₂SO₄  848

From Table 1, it is considered that sulfates of alkali metals such asLi, Na, K, Rb, Cs, etc, alkaline earth metals such as Mg, Ca, Sr, Ba,etc and Ce, Al, Y, La, Ni, etc have high melting point or decompositiontemperatures and they are suitable as the sulfur component trappingagents.

When the sulfur component trapping agent is the alkali metals oralkaline earth metals, for example, sulfates of the alkali metals oralkaline earth metals are produced by reaction according to the formula(1) when SOx contained in the exhaust gas in the lean air fuel ratiocontacts with the sulfur component trapping agents.

As is discussed above, when the noble metals are present in the sulfurcomponent trapping agents, release of sulfur components tends to occurat the time of rich air fuel ratio. Accordingly, the noble metals suchas Rh, Pt, Pd, etc should preferably be not contained in the sulfurcomponent trapping agents. A total amount of Rh+Pt+Pd should be 0.4% byweight or less per a weight of the sulfur component trapping agent. Anamount of the sulfur component trapping agents depends on kinds of fuelsused and required performance.

Estimation on the total sulfur components exhausted from a diesel carfor a running at a speed of 16000 km was made. If a fuel consumption is20 km/L, an amount of light oil is 8000 L. On the other hand, if aspecific gravity of the light oil is 0.85 g/cc and a concentration ofsulfur components in the light oil is 10 ppm, a total sulfur exhaustedis 2.1 moles. Accordingly, in order to trap all of the sulfur, 4.2 molesof Na₂CO₃ is required thereby to convert Na₂CO₃ to Na₂SO₄, when Na₂CO₃is used as the sulfur component trapping agent.

On the other hand, when CaO is used as the sulfur component trappingagent, 2.1 moles of CaO is required to convert CaO to CaSO₄.Accordingly, when the alkali metals are used, at least 4.2 moles arenecessary, and when the alkaline earth metals are used, at least 2.1moles the alkali metals are necessary.

Although only the sulfur trapping components can be used in the sulfurcomponent trapping agent, the sulfur trapping components may besupported on a porous support. The porous support may function toenhance dispersion of the sulfur trapping components. When the poroussupport is used, an amount of the sulfur trapping component, which is0.2 mole part or less in a metallic element conversion per 1.9 moleparts of the porous support, is insufficient for sulfur componenttrapping performance if a large amount of sulfur components isgenerated. If the amount of the sulfur trapping component is 0.8 molepart or larger, the sulfur component trapping agent tends to cohere onthe porous support thereby to decrease the sulfur trapping capacity.Accordingly, an amount of the sulfur trapping components is preferably0.2 to 0.8 mole part per 1.9 mole parts of the porous support.

In the specification, the words “mole part” are used to representconcentration rates of respective components in conversion of moles; ifa supported amount of component B is 0.5 mole part per 1 mole part ofcomponent A, a mole ratio of the amount of B component to that of Acomponent is 0.5, irrespective of an absolute amount of A component.

The porous support may be supported on a substrate; in this case, apreferable supporting amount of the support is 30 to 400 g per 1 L ofthe substrate for the sulfur component trapping performance. If thesupporting amount of the porous support is less than 30 g, theperformance of the porous support will be insufficient, and if thesupporting amount of the porous support is larger than 400 g, thespecific surface area of the porous support itself becomes small. Theabove are not preferable.

If the amount of the porous support is too small, a sulfur trappingamount becomes small, and if the amount id too large, the specificsurface area of the porous support becomes small.

As the porous supports, there are metal oxides or composite oxides ofalumina, titania, silica, silica-alumina, zirconia, magnesia, etc.Alumina is particularly suitable because it is good in heat resistanceand has a function of good dispersion of the sulfur trapping componentsthereon.

Various shapes of the sulfur component trapping agents and sulfuroxidizing catalysts may be employed in accordance with applications.Honeycomb structures produced by coating the sulfur component trappingagent and the sulfur component oxidizing agent on a honeycomb substratemade of cordierite, SiC, stainless, etc, or pellets, plates, granules,powders may be used. In the case of honeycomb structures, cordierite isthe most preferable. However, if there is a possibility of temperatureelevation of the catalyst, metallic substrates, which hardly react withthe catalysts, would bring about good results. Further, it is possibleto produce honeycombs made of only the sulfur component trapping agentsand the sulfur component oxidizing agents.

(Investigations on NOx Purification Catalyst Components)

Any NOx trapping catalysts that trap and purify NOx can be used; NOxtrapping catalysts comprise at least one selected from alkali metals andalkaline earth metals and a noble metal, which trap NOx at highefficiency and high NOx purification performance.

Methods for preparing the catalysts include physical preparation methodsand chemical reaction preparing methods such as impregnation methods,mixing methods, co-precipitation methods, sol-gel methods, ion-exchangemethods, evaporation methods, etc.

As starting materials for the exhaust gas purification catalysts,various compounds, metals and metal oxides such as nitrate compounds,chelate complex compounds, hydroxides, carbonates, organic compounds,etc can be used.

(Investigations on Trapping Amounts in Rich Air Fuel Ratio)

Most of sulfur components in the exhaust gas are present in a form ofSOx when the exhaust gas is in a lean air fuel ratio. On the other hand,when the air fuel ratio is rich, it is conceived that the sulfurcomponents are present mainly as H₂S. Since a period of the rich airfuel ratio is shorter than that of the lean air fuel ratio, a largereduction in trapped sulfur components is not required unless thetrapped sulfur components are released in the lean air fuel ratio. Thereduction rate is defined as an amount of sulfur present after thesulfur trapping agent/an amount of sulfur flowing into the sulfurtrapping agent×100 (%).

When sulfur components are present in the form of H₂S, there is a casewhere degradation of NOx trapping catalysts by sulfur components issmall. Considering that the NOx trapping catalysts are degraded by thereaction represented by the formula (1), degradation of the NOx catalystis less problematic even if the reduction in the trapped sulfurcomponents is low, when the amount of the sulfur components that enterthe NOx trapping catalysts at the time of rich air fuel ratio is smallis lowered.

Accordingly, if there is almost no lean air fuel ratio condition in arunning mode of the internal combustion engine, it is possible to reducean amount of sulfur component trapping agents of the exhaust gaspurification apparatus thereby to lower the cost of the apparatus and tosave space thereof.

Such sulfur component trapping agents as to trap 60% of the sulfurcomponents entering the sulfur component tripping agents are preferablyused. The trapping rate of 85% or more is more preferable. That is, whena temperature of the sulfur component trapping agent is set to 300° C.and gas consisting of 150 ppm H₂S-0.5% O₂—balance being N₂ is flown at aspace velocity SV of 30,000/h for 1 hour, a sulfur component trappingagent having a trapping rate of 60% or more should preferably be used;85% is more preferable.

The present invention provides an internal combustion engine providedwith the sulfur component trapping agent. The above-mentioned DPF isprovided with a sulfur component trapping agent as an example fortrapping sulfur components. Further, an internal combustion engineequipped with the above-mentioned sulfur component trapping agent andthe catalyst in the exhaust duct is operated under a lean air fuelratio, and then the operation is switched to a rich or stoichiometricair fuel ratio, followed by switching to lean air fuel ratio to performpurification of the gas. If poisoning of the NOx trapping catalystproceeds, a temperature at the entrance of the NOx trapping catalyst iselevated and gas of a stoichiometric or rich air fuel ratio is flownthereby to release sulfur components from the NOx trapping catalyst andregenerate the catalyst.

EMBODIMENTS

In the following embodiments of the present invention will be described.The present invention is not limited to these embodiments.

(Preparation Methods of NOx Catalyst)

After a slurry containing alumina powder and a precursor of aluminaadjusted with nitric acid was coated on a cordierite honeycomb (400cells/in²) and the coating was dried thereby to produce analumina-coated honeycomb with an amount of alumina of 1.9 moles per 1 Lof an appearance volume.

After the alumina-coated honeycomb was impregnated with a firstimpregnation component containing cerium nitrate, the honeycomb wasdried at 120° C., followed by calcining it at 600° C. for 1 hour. Thenthe Ce carrying honeycomb was impregnated with a second impregnationcomponent mixture containing dinitrosodiamine platinum nitrate solution,dinitrosodiamine palladium nitric acid solution, rhodium nitratesolution and potassium acetate, and the honeycomb was dried at 200° C.,followed by calcining it at 600° C. for 1 hour. Then, the honeycombimpregnated with Ce, Rh, Pt, Pd and K was impregnated with a thirdimpregnating component mixture containing potassium acetate, sodiumnitrate, lithium nitrate and titanium sol. Thereafter, the honeycomb wasdried at 200° C., followed by calcining it at 600° C. for 1 hour. Thepotassium acetate in the second and third impregnation mixture was thesame. At the final step, the honeycomb was treated in an electric ovenat 700° C. for 5 hours.

According to the above-mentioned procedure, a NOx trapping catalyst 1was prepared wherein the composition was 190 g of alumina, 27 g of Ce,12.4 g of Na, 15.6 g of K, 0.4 g of Li, 4.3 g of Ti, 0.139 g of Rh,2.792 g of Pt and 1.35 g of Pd, in conversion of elements.

(Preparation of Sulfur Component Trapping Agent)

Sulfur component trapping agents were prepared from Li, Na, K, Cs, Mg,Ca, Sr, Ba, La and Fe, respectively. The alkali metals were used in theform of carbonates, alkaline earth metals were in the form of oxides,and La₂O₃ and FeO. Amounts of Li, Na, K, Cs, Mg, Ca, Sr, Ba, La and Fewere 1.5 moles per 1 liter of the honeycomb. In accordance with the samemanner as in the preparation of the NOx trapping catalyst 1, sulfurcomponent trapping agents were prepared wherein the above componentswere impregnated in the cordierite honeycombs to produce sulfurcomponent trapping agents A, B, C, D, E, F, G, H, I and J.

Embodiment 1 Installation of Sulfur Component Oxidizing Catalyst

As shown in FIG. 1, the sulfur component trapping agent B (Na₂CO₃) wasinstalled before the NOx trapping catalyst 1 in an engine exhaust gasduct. Further, a sulfur component oxidizing catalyst was disposed beforethe sulfur component trapping agent to constitute a purificationapparatus. As the sulfur component oxidizing catalyst, Al₂O₃ with Ptprepared in the same manner as in the method for preparing the NOxtrapping catalyst was used. The sulfur component oxidizing catalystcomprises 190 g of alumina per 1 liter of the honeycomb, 2.792 g of Pt,in conversion of elements.

In FIG. 1, a comparative embodiment 1 had no sulfur component oxidizingcatalyst. SOx containing lean gas was supplied to the sulfur componenttrapping agents to evaluate sulfur trapping rates by the sulfurcomponent trapping agents.

Temperatures of the sulfur component oxidizing catalysts and sulfurcomponent trapping agents were 400 degrees and 300° C., respectively. Aflow rate of Sox containing lean gas was 3 liters/min and concentrationof SO₂ was 150 ppm. The flow time was 1 hour.

The composition of the lean gas supplied to the sulfur componenttrapping agents is shown in Table 2. The sulfur component trapping rateswere calculated by the equation (6) below.Sulfur component trapping rate (%)=(Sulfur amount of sulfur componentstrapped by the sulfur component trapping agent, mole)/an inflow amountof sulfur to the Sulfur component trapping agent, mole)×100(%)  (6)

TABLE 2 Lean gas composition Gas components Gas concentration (%) Gasamount (mmol/h) O₂ 5 401.8 N₂ balance balance SO₃ 150 ppm 1.215

FIG. 2 shows sulfur component trapping rates in cases where the sulfurcomponent oxidizing catalyst was disposed and was not. It is apparentfrom FIG. 2 that the disposition of the sulfur component oxidizingcatalyst exhibits higher sulfur trapping rates with the sulfur componenttrapping agent.

As is disclosed in Japanese patent laid-open 11-169708, it has beenknown that Pt functions as a sulfur component oxidizing catalyst.Accordingly, when the sulfur component oxidizing catalyst is disposed,SO₂ oxidation reaction takes place as represented by the formula (4),and the sulfur components entering the sulfur component trapping agentare mainly SO₃. Since the bonding force of SO₂ to the sulfur componenttrapping agent is weaker than that of SO₃, SO₂ is hardly trapped. Thus,sulfur components in the form of SO₃ are more easily trapped than in theform of SO₂ gas. Accordingly, the disposition of the sulfur componentoxidizing catalyst before the sulfur component trapping catalystimproves the sulfur component trapping rates, as is apparent from FIG.2.

Embodiment 2 Evaluation of Sulfur Component Trapping Performance of theSulfur Component Trapping Agent

In the system shown in FIG. 1, amounts of sulfur trapped by the sulfurcomponent trapping agent were calculated in a thermodynamic method whenSOx containing lean gas was supplied to the sulfur component trappingagent. As calculation software, MALT2 (thermodynamic database forpersonal computer; Japan Society of thermodynamics) was used.

A temperature of the sulfur component trapping agent was 300° C. A flowrate of the SOx containing lean gas was 3 L/min. The sulfur componentsin the gas were oxidized to SO₃ by the sulfur component oxidizingcatalyst disposed before the sulfur component trapping agent. Thus, aconcentration of SO₃ was estimated as 150 ppm. A period of flow time was1000 hours. Gas components other than SO₃ were shown in Table 3. Thesulfur trapping rates were calculated by the equation (6). TABLE 3Sulfur trapping rates of the sulfur component trapping agents A, B, C,D, E, F, G, H, I and J. Sulfur component Sulfur component trapping agenttrapping rate (%) A Li₂CO₃ 100 B Na₂CO₃ 100 C K₂CO₃ 100 D Cs₂CO₃ 100 EMgO 100 F CaO 100 G SrO 100 H BaO 100 I La₂O₃ 100 J FeO 0

From the above table, it is apparent that when the sulfur componenttrapping agents A to I are used, the sulfur trapping rates exceed 85%,which are sufficiently high. Accordingly, it is possible to sufficientlysuppress the sulfur components entering the NOx trapping catalyst 1disposed after the sulfur component trapping agent. Further, the sulfurcomponent trapping agents A to I were converted into sulfates of alkalimetals, alkaline earth metals and La₂O₃ by virtue of trapping sulfurcomponents. The sulfur trapping reactions of the alkali metals andalkaline earth metals with sulfur components may be represented by thefollowing formulae.M₂CO₃+SO₂+1/2O₂→M₂SO₄+CO₂  (7)

(M: alkali metal)M′O+SO₂+1/2O₂→M′SO₄  (8)

(M′: alkaline earth metal)

The melting points or decomposition temperatures of the sulfates ofalkali metals, alkaline earth metals and La sulfate are higher than 750°C. ; thus, once the sulfates are formed, they are not decomposed unlessthe temperature becomes above 750° C. Accordingly, if the temperature ofthe sulfur component trapping agents is below 750° C., the sulfurcomponents trapped by they are not released again even when a normaltemperature rise of the exhaust gas.

Embodiment 3 Evaluation on Decomposition Sulfates of Sulfur ComponentTrapping Agents

Decomposition of sulfates of the sulfur component trapping agents wasevaluated. Sulfates/Al₂O₃ prepared by drying and mixing various sulfatesand Al₂O₃ were used. A concentration of the sulfur component trappingagents per 10 grams of alumina was 0.04 mol in conversion of elements.

As samples of sulfates of the sulfur component trapping agents, sulfatesof alkali metals, alkaline earth metals, and sulfates of Ce, Al, La, Yand Ni were chosen.

Powders of the above sulfur component trapping agents were granulatedinto 0.85 to 1.70 mm diameter. The temperature of the sulfur trappingagents was kept at 300° C. and the rich gas shown in Table 4 was flownthrough the trapping agents, while elevating a temperature of from 250to 750° C. Concentrations of sulfur components (SO₂+H₂S) released fromthe trapping agents were measured. A space velocity SV of the gas was30,000/h. TABLE 4 Gas composition Composition (Rich air fuel ratio) N₂Balance H₂ 3000 ppm CO 3.5% O₂ 3000 ppm C₃H₆ 600 ppm

Decomposition performance of the sulfur component trapping agents wascalculated in accordance with the following equation.Sulfur component release rate (%)=(an amount of sulfur released until750° C. (mol)/(an amount of sulfur trapped by the trapping agentmol)×100(%)  (9)

The results are shown in Table 5 below. TABLE 5 Evaluation ondecomposition performance of sulfates of sulfur trapping agents Sulfurtrapping Sulfur component agent Sulfate release rate (%) Li Li₂SO₄ 1 NaNa₂SO₄ 0 K K₂SO₄ 0 RB Rb₂SO₄ 0 Cs Cs₂SO₄ 0 Mg MgSO₄ 2.5 Ca CaSO₄ 2.5 SrSrSO₄ 2 Ba BaSO₄ 1.5 Ce Ce₂(SO₄)₃ 4 Al Al₂(SO₄)₃ 4.5 La La₂(SO₄)₃ 3 YY₂(SO₄)₃ 3 Ni NiSO₄ 4.5

It is apparent from Table 5 that the sulfur component release rates ofthe sulfur component trapping agents are 5% or less; the sulfurcomponents trapped by the sulfur component trapping agents are hardlyreleased and the trapping agents are suitable for the sulfur componenttrapping agents.

Embodiment 4 Influence of Coexistence of Noble Metals

Influence of coexistence of noble metals and the sulfate (Na₂SO₄) in thetrapping agent B on decomposition of the sulfate was evaluated.

Na₂SO₄ was impregnated with a Pd solution, and the impregnated Na₂SO₄was dried at 160° C., followed by calcining it at 600° C. for 1 hour.The resulting was mixed with dried Al₂O₃ to prepare a Pd containingcatalyst (Na₂SO₄—Pd/Al₂O₃). A catalyst (Na₂SO₄/Al₂O₃) was prepared inthe same manner as in the above, except for not containing Pd. The abovecatalysts were used in the evaluation. Concentration of Na and Pd were0.04 mol of Na and 0.15 g of Pd per 10 g of Al₂O₃, in conversion ofelements.

Powders of the above catalysts were granulated into 0.85 to 1.70 mm indiameter. The temperature of the catalysts was kept at 300° C. and richgas was flown through the catalysts, while elevating temperatures from250 to 800° C. to measure amounts of sulfur components (SO₂+H₂S)released from the catalysts. The space velocity SV of the gas was30,000/h.

FIG. 3 shows the results. In the case of Pd impregnation, when thetemperature of the catalysts exceeds 500° C., release of sulfurcomponents was observed. This means decomposition of the sulfates. Asthe temperature of the catalysts elevates, amounts of released sulfurcomponents increased; the amount of released sulfur components around750° C. was over 200 ppm.

On the other hand, in the case of the catalyst not containing Pd,release of sulfur component was not observed when the temperature waselevated to 800° C. In the case of Pd impregnation, an amount ofreleased sulfur until 800° C. was 9% per an amount of sulfur, which wasoriginally contained in the catalyst. From the above fact, when Pd isimpregnated, decomposition of sodium sulfate takes place at atemperature of 500° C. or higher.

If the noble metals and the sulfur component trapping agents are incontact, release of the trapped sulfur components occurs when the richgas flows thereby to bring about poisoning of the NOx trapping catalyst1. Accordingly, it is preferable that the sulfur component trappingagents do not contain noble metals.

Embodiment 5 Influence of Coexistence of Noble Metals

Decomposition performance of lithium sulfate using a sulfur componenttrapping agent comprising LiPt/Al₂O₃, which was prepared in the samemanner as of the NOx trapping catalyst, was evaluated. Additive amountswere 190 g of alumina per 1 L of the honeycomb, 1.8 g of Li and 2.792 gof Pt, in conversion of elements.

A temperature of the LiPt/Al₂O₃ catalyst was set to 300° C., and a leanmodel gas whose composition is shown in Table 6 was flown for 1 hourthereby to cause the catalyst to absorb sulfur components. Consideringthat the sulfur component trapping agent is installed in the systemshown in FIG. 1, the sulfur components to be flown through the trappingagent LiPt/Al₂O₃ was SO₃.

While the rich gas whose composition is shown in Table 6 was being flownthrough the catalyst, the temperature of the gas was elevated from 300to 700° C. ; then concentrations of SO₂ released from the LiPt/Al₂O₃catalyst were measured. The space velocity SV of the gas was 30,000/h atlean and rich conditions.

FIG. 4 shows the results. A peak is observed around 400° C. It isassumed that the decomposition reaction of sulfates takes place aroundthat temperature.

On the other hand, decomposition temperatures of Li₂SO₄ and Al₂(SO₄)₃are 1256° C. and 770° C., respectively. Accordingly, it is conceivablethat the decomposition of sulfate, which took place at 400° C., wascaused by Pt added to the catalyst. That is, when the noble metals andthe sulfur component trapping agents are in contact, release of trappedsulfur components takes place at the time of flowing the rich gasthereby to bring about poisoning of the NOx trapping catalyst.Accordingly, the sulfur component trapping agents should not contain thenoble metals. TABLE 6 Gas composition Gas composition Lean Rich N₂balance balance CO₂ 10% 12% H₂O 10% 10% H₂ 0 ppm 3000 ppm CO 1000 ppm6000 ppm O₂  5% 5000 ppm C₃H₈ 500 ppm 600 ppm SO₃ 150 ppm 0 ppm NO 600ppm 1000 ppm

Embodiment 6 Quantitative Influence of Coexistence of Noble Metals

A KNaRhPtPd/Al₂O₃, which was prepared in the same manner as in themethod of preparing the NOx trapping catalyst 1, was used as the sulfurcomponent trapping agent, and its sulfur component release performancewas evaluated. Compositions of the trapping agents were 190 g of aluminaper 1 L of the honeycomb, 12.4 g of Na and 15.6 g of K, in conversion ofelements, and noble metals (Rh+Pt+Pd), wherein the total amounts ofnoble metals were 0% by weight, 0.3% by weight and 0.7% by weight perthe weight of the sulfur component trapping agent. Weight ratios of Rh,Pt and Pd were 1:20:10.

After the entrance temperature of the KNaRhPtPd/Al₂O₃ was set to 300°C., the model gas shown in Table 7 was fed though the sulfur componenttrapping agent for 2 hours, then the temperature was elevated to 650°C., followed by letting the rich model gas flow for 10 min. Sulfurrelease rates were measured.

Considering that the sulfur component trapping agent is installed in thesystem shown in FIG. 1, the sulfur components entering the sulfurcomponent trapping agent in lean air fuel ratio was SO₃. The spacevelocity SV of the gas in lean and rich periods was 30,000/h. TABLE 7(Gas composition) Gas composition Lean rich N₂ Balance balance CO₂ 10%12% H₂O 10% 10% H₂ 0 ppm 3000 ppm CO 1000 ppm 3.5%  O₂  5% 3000 ppm C₃H₆500 ppm 600 ppm SO₃ 300 ppm 0 ppm NO 600 ppm 1000 ppm

The sulfur component release rates were calculated in accordance withthe equation (10).Sulfur component release rate (%)=An amount of sulfur released from thesulfur trapping agent in a rich period (mol)/an amount of sulfurreleased from the sulfur trapping agent in a lean period(mol)×100(%)  (10)

FIG. 5 shows the results. When the amount of Rh+Pt+Pd is 0.4% by weightor less, the release rate of sulfur component is less than 2%, and thesulfur component trapping agent exhibits a high sulfur componentretention performance. Accordingly, in order to expect a high sulfurcomponent retention performance, the total amount of Rh+Pt+Pd should be4% by weight or less.

Embodiment 7 Support on a Filter

As a method of arranging the sulfur component oxidizing catalysts beforeand after the sulfur component trapping agent, a filter can be utilized.As the filter, there may be exemplified diesel particulate filters(DPF), which have been used for removing particle material (PM).

For example, the sulfur component oxidizing catalyst is supported on asurface of the upper stream side of the DPF and the sulfur componenttrapping agent is supported on a surface of the downstream side of thefilter. By this arrangement, it is possible to oxidize the exhaust gasto form SO₃ when the exhaust gas enters DPF so that the SO₃ can betrapped by the sulfur component trapping agent. In this case, if thenoble metals are used as the sulfur component oxidizing catalyst,decomposition of sulfates of the sulfur component trapping agent doesnot take place and flowing-out of the trapped sulfur does not occur evenwhen the air fuel ratio becomes rich, because the noble metals are notin contact with the sulfur component trapping agent.

When the sulfur component oxidizing catalyst and the sulfur componenttrapping agent are disposed with a distance, there may be a problem thatthe formed SO₃ may adhere to a wall of the gas passage thereby to damagethe gas passage; on the other hand, in the case of filters, the aboveproblem may be removed, because the formed SO₃ is immediately trapped.Since the both faces of the filters can be utilized, the space can bereduced, compared with the case where the sulfur component oxidizingcatalyst and the sulfur component trapping agent are separatelyarranged.

The present invention can be applied to diesel exhaust gas, wherein PMcontained in the gas includes solid substance such as soot, sulfates andmists, which contain sulfur components. When the filters are used, thesesolid and liquid sulfur components as well as sulfur components in thegas phase are removed thereby to remarkably increase trappingperformance of the sulfur component trapping agent.

Any filters may be employed as long as they perform the above functions.Materials for the filters may include cordierite, stainless steel, SiC,etc. Various shapes of the filters may be employed. For Example, crosssections thereof may be circular, rectangular, elliptic, etc.

The sulfur component oxidizing catalyst and sulfur component trappingagent may be supported on filters by any proper manners such as whollyor partially supported on the filter surface in accordance with objectsexpected.

FIG. 6 shows a sectional view of a DPF (diesel particulate filter),which employs the sulfur component oxidizing catalyst and the sulfurcomponent trapping agent. The sulfur component oxidizing catalyst issupported on the upstream side face of the filter and the sulfurcomponent trapping agent is supported on the downstream side face of thefilter. When exhaust gas enters DPF, it is oxidized by the sulfurcomponent oxidizing catalyst to form SO₃ and the resulted SO₃ iscontacted with the sulfur component trapping agent. When the noblemetals are used as the sulfur component oxidizing catalyst, the sulfatesof the sulfur component trapping agent by the noble metals in the richair fuel ratio condition does not occur and release of sulfur componentsdoes not occur as well, because the noble metals do not contact with thesulfur component trapping agent.

When the sulfur component oxidizing catalyst and sulfur componenttrapping agent are disposed separately, produced SO₃ adheres to thepassage walls thereby to degrade the passage material. However, in theabove-mentioned filter PDF there is no problem because the formed SO₃ isimmediately trapped.

Since the both faces of the filter are utilized, the space for thefilter is smaller than the case where the catalyst and the trappingagent are separately disposed.

The exhaust gas from the diesel engines contains solid matters such assoot, sulfates, etc and mists. These substances contain the sulfurcomponents. Since the filters can remove these solid or liquid mattersas well as sulfur components in the gas phase, the sulfur componenttrapping performance is remarkably improved.

Embodiment 8 Installment of the Sulfur Trapping Agent Just Below anEngine

FIG. 7 shows a diagrammatic view of an arrangement where the honeycombtype sulfur component trapping agent is disposed just below the engine.The words “just below the engine” are used to mean a position which isas close to the engine as possible. For example, the position is within1 m from an exhaust manifold entrance port. The NOx trapping catalystmay be disposed vertically.

In disposing the sulfur component trapping agent beneath a floor, aspace is necessary in the floor. On the other hand, in disposing thesulfur trapping agent below the engine the space in the floor is notnecessary so that a car room becomes wide.

The exhaust gas from an engine contain, which condenses. If the sulfatesare formed on the sulfur component trapping agent, the sulfates dissolveinto condensed water, which may be discharged as sulfuric acid into thedownstream. This phenomenon leads to corrosion of the passage anddegradation of the NOx trapping catalyst disposed after the sulfurcomponent trapping agent.

When the sulfur component trapping agent is disposed just below theengine, a temperature of the sulfur component trapping agent tends toelevate thereby to prevent condensation of water. When the sulfurcomponent trapping agent disposed just below the engine is a honeycombtype structure, the condensed water does not stay in the honeycombstructure to prevent the above problem, because a direction of the gaspassages is vertical the ground.

Embodiment 9 Estimation of S Purge Frequency

It is difficult to completely prevent sulfur components from enteringthe NOx trapping catalyst disposed after the sulfur component trappingagent. Accordingly, degradation of performance of the NOx trappingcatalyst to which the sulfur components adhere after a lapse of a longperiod of time even when the sulfur component trapping agent isdisposed; thus the NOx trapping catalyst should be subjected to S purge.

An SO₂ containing model lean gas whose composition is shown in Table 6was flown through the NOx trapping catalyst 1 so as to evaluatedegradation of the catalyst by sulfur components. The temperature at theentrance of the catalyst was set to 300° C., and the space velocity ofthe gas was 30,000/h. FIG. 8 shows degradation rates of catalyticactivity of the catalyst with respect to treatment time. The NOxpurification rate was calculated by the following equations.NOx purification rate (%)={(an amount of NOx entered the catalyst within1 min. after switching to lean condition)−(an amount of NOx flowing-outfrom the catalyst within 1 min. after switching to lean condition)}/anamount of NOx flowing-out from the catalyst within 1 min. afterswitching to lean condition)×100  (11)

From the above results, it is apparent that when the SO₂ containing leanmodel gas is flown through the catalyst for 1 hour, the catalyticactivity lowers to 40%, which necessitates S purge.

When a lean model gas containing 300 ppm of SO₂ is flown through thecatalyst for 1 hour, an amount of entering sulfur components is 2.4mmoles. Estimation of an amount of sulfur components entering thecatalyst in a conventional exhaust gas purification apparatus is madeunder conditions that a concentration of sulfur components in gasolineis 10 ppm and fuel consumption is 10 km/L, the above entering amount ofsulfur components is equivalent to a running for 90.6 km. Accordingly,one S purge is required for every 90.6 km running.

If the S concentration in the exhaust gas is lowered by the sulfurcomponent trapping agent in the system shown in FIG. 1 to thereby lowerthe S concentration of the gas entering the catalyst 1 to 1.5 ppm, the Spurge is required for every 9060 km. In accordance with the abovetheory, relationship between calculation results of trapping rates ofsulfur components in lean condition and frequency of S purge is shown inFIG. 9. The sulfur component trapping rate was calculated by theequation (6). The S purge frequency represents a distance between two Spurges.

From FIG. 9, it is apparent that when the S trapping rate exceeds 85%,the S purge frequency exceeds 604 km. Accordingly, this value is 6 timesor more of that where the sulfur component trapping agent is notdisposed. Thus, the advantage of the sulfur component trapping agent isremarkable.

Embodiment 10 Evaluation of Poisoning Depending on Gas Atmosphere

Lean gas and rich gas whose compositions are shown in Table 8 were flownthrough the NOx trapping catalyst 1 for 1 hour. A temperature at theentrance of the catalyst was 300° C. and the space velocity was30,000/h. In the lean gas the sulfur components present in the form ofSO₂, and in the rich gas the sulfur components present in the form ofH₂S. TABLE 8 Released gas from NOx trapping catalyst Gas compositionlean Rich N₂ balance balance CO₂ 10% 12% H₂O 10% 10% H₂ 0 ppm 3000 ppmCO 1000 ppm 3.5%  O₂  5% 3000 ppm C₃H₆ 500 ppm 600 ppm NO 600 ppm 1000ppm SO₂ 150 ppm 0 ppm H₂S 0 ppm 150 ppm

FIG. 10 shows amounts of sulfur adhered to the NOx trapping catalyst 1.When the catalyst is poisoned by SO₂ (lean gas), the amount of adhesionof S is about 4 times that in case of poisoning by H₂S (rich gas).Accordingly, poisoning by the lean gas goes faster than in the case ofthe rich gas.

Suppose the sulfur component trapping agent is used in the system shownin FIG. 1. If a sulfur component trapping rate is 90%, 10% of sulfurcomponents outflows from the sulfur component trapping agent. In view offacts that 90% of the sulfur component trapping arte is sufficient andthe S poisoning by lean gas is 4 times that by rich gas, (100−10×4=60%)or more of the trapping rate of the S component trapping trapped by thesulfur component trapping agent in case of rich gas is preferable.

Embodiment 11 Spurge

Elevation of temperature of the exhaust gas is necessary at the time ofS purge. If the elevation is too much and the temperature exceeds themelting point or decomposition temperature of the sulfates, release ofsulfur from the sulfur component trapping agent takes place.Accordingly, S purge should be carried out at a temperature as low aspossible. For example, the S purge should preferably be carried out at atemperature of 500 to 700° C. at the NOx trapping catalyst entrance.

In the arrangement shown in FIG. 1, used were the Pt added Al₂O₃catalyst in Embodiment 1 as the sulfur component oxidizing catalyst andthe trapping agent B as the sulfur component trapping agent. Under thesame conditions as in embodiment 8, the NOx trapping catalyst 1 wassubjected to S purge in the rich gas flow after the activity of the NOxtrapping catalyst was decreased to 40%. The rich gas whose compositionis shown in Table 6 was supplied for 10 min to the NOx trapping catalyst1 of which activity was decreased to 40% at a temperature of 300° C. atthe NOx trapping catalyst entrance. A temperature of the NOx trappingcatalyst during the rich gas flow was 650° C.

FIG. 11 shows the results. The NOx purification rate was calculated bythe equation (11). Activity of the NOx trapping catalyst 1 wasregenerated by flowing rich gas for 10 min to the activity before supplyof the sulfur components.

In view of the fact that in embodiment 3, decomposition of the trappingagent B did not occur until 800° C. in addition to the above fact, it isapparent that release of the sulfur components from the sulfur componenttrapping agent does not occur thereby to regenerate the NOx trappingcatalyst, when the temperature of the sulfur component trapping agent isset to 750° C. and the temperature of the NOx trapping catalyst 1 is setto 650° C. at the time of S purge.

Embodiment 12 Diagnosis of Degradation of Sulfur Component TrappingAgent

Since the sulfur component trapping agent of the present invention isnot one that is used and regenerated as in the conventional sulfurtrapping materials, it is necessary to replace the sulfur componenttrapping agent with new one or a part of the exhaust gas purificationapparatus is replaced. The present invention provides a system forindicating information on timing of replacement of the sulfur componenttrapping agent, wherein the degree of degradation of the sulfurcomponent trapping agent is estimated by the following method. When thesulfur component trapping agent degrades, degradation of the NOxtrapping catalyst disposed after the sulfur component trapping agentproceeds. Therefore, there is a correlation between the degree ofdegradation of the sulfur component trapping agent and poisoning speedof the NOx trapping catalyst by sulfur.

When S purge is applied to the NOx trapping catalyst, the degree ofregeneration of activity of the NOx trapping catalyst greatly poisonedby sulfur is large. Accordingly, the degradation of the NOx trappingcatalyst by sulfur can be estimated by the degree of regeneration ofactivity caused by S purge, and from that result degree of degradationof the sulfur component trapping agent can be estimated.

That is, when NOx purification rates after regeneration and beforeregeneration of the catalyst are measured and when a difference or aratio between them becomes over a certain value, there is provided anapparatus for diagnosing degradation that indicates replacement of thesulfur component trapping agent and a replacement supporting system.

In order to make the replacement easy, the sulfur component trappingagent and a part of the NOx purification catalyst may be madedetachable. According to the embodiment, the exhaust gas purificationapparatus of the present invention can be used for a long time keepingeffective purification performance.

Embodiment 13 Constitution of Internal Combustion Engine

FIG. 12 shows a schematic diagram of an embodiment of an internalcombustion engine equipped with the exhaust gas purification apparatusof the invention.

The exhaust gas purification apparatus of the invention comprises anengine 99, which is capable of lean burn combustion, an air-intakesystem comprising an air flow sensor 2, a throttle valve 3, etc; anexhaust gas system comprising an oxygen concentration sensor or A/Fsensor 7, a gas temperature sensor 8 for a NOx trapping catalystentrance, a gas temperature sensor 9 for a sulfur component trappingagent entrance, a temperature sensor 10 for a sulfur component trappingagent, the sulfur component trapping agent 12, a NOx trapping catalyst13, a sulfur component oxidizing catalyst 14, a temperature sensor 15for the sulfur component oxidizing catalyst, a temperature sensor 16 forthe sulfur component oxidizing catalyst entrance, etc, a control unit(ECU) 11, etc. ECU is constituted by I/O for an interface of input andoutput, LSI, a calculation device MPU, memory devices RAM and ROM thatmemory a large number of control programs, a timer counter, etc.

The above-mentioned exhaust gas purification apparatus works as follows.An intake air amount into the engine is measured by the air flow sensor2 after filtering with an air cleaner 1′. It flows through the throttlevalve 3 and an injector 5 where the air is injected with fuel; then themixed gas is supplied to the engine 99. Signals from the air flow sensorand other sensors are input into the engine control unit ECU.

Running conditions of the internal combustion engine and conditions ofthe NOx trapping catalyst, etc are evaluated by ECU to decide air fuelratio and injection time, etc of the injector 5 are controlled toconcentration of the fuel to a predetermined values. The mixed gasintaken into cylinders is ignited by ignition plugs 6, which arecontrolled by signals from ECU 11 to combust air/fuel mixture.

Combustion exhaust gas is introduced into an exhaust gas purificationsystem. The exhaust gas purification system is provided with the exhaustgas purification catalyst 13 for lean burn combustion, wherein NOx, HCand CO in the exhaust gas are purified by three way function of thecatalyst in stoichiometric combustion running. In lean combustionrunning, NOx is purified by its NOx trapping function and HC and CO arepurified by burning them by the catalyst. When the exhaust gas containsSOx, the sulfur components are oxidized by the sulfur componentoxidizing catalyst and then almost all of the oxidized sulfur componentsis removed, followed by introducing the gas into the catalyst 13. If thecatalyst 13 is partially poisoned by Sox to lower the NOx trappingperformance, the NOx trapping performance of the catalyst 13, which isnormally monitored during the lean combustion is recovered by shiftingthe air fuel ratio of the gas to a rich side in response to judgingsignals and control signals from ECU. At the same time, the temperatureof the catalyst 13 may be elevated in response to the judging signalsand control signals from ECU to cause the sulfur components to releasefrom the catalyst 13 so that the catalyst 13 is regenerated.

According to the above-described operation, exhaust gases over theentire operation conditions including lean and stoichiometric (includingrich) operations are effectively purified.

1. An exhaust gas purification apparatus disposed in an exhaust gaspassage of an internal combustion engine having a NOx purificationcatalyst, which comprises a sulfur component trapping agent for trappingsulfur components, which is arranged before the NOx trapping catalystand a catalyst for oxidizing the sulfur components, disposed before thesulfur component trapping agent, wherein the sulfur component trappingagent does not substantially release the trapped sulfur components underthe conditions of the internal combustion engine.
 2. An exhaust gaspurification apparatus for an internal combustion engine, whichcomprises an exhaust gas passage for an internal combustion engine intowhich exhaust gas of lean air fuel ratio and rich or stoichiometric airfuel ratio flows, a NOx trapping catalyst that functions to trap NOx inthe exhaust gas when the air fuel ratio is lean, a sulfur componenttrapping agent for trapping sulfur components in the exhaust gas, whichis disposed before the NOx trapping catalyst, and a catalyst foroxidizing the sulfur components, which is disposed before the sulfurcomponent trapping agent, wherein the sulfur component trapping agenthas a trapping rate of 85% or more of an amount of inflow sulfur in atrapping test at a flow rate of 150 ppm SO₃-5% O₂—balance being N₂ gasper 1.5 moles of the sulfur component trapping agent at 300° C. and aspace velocity of 30,000/h for 1 hour; and the sulfur component trappingagent has a release rate of sulfur amount of 5% or less of sulfurtrapped in the sulfur component trapping agent in a release test under aflow of a 3000 ppm H₂—600 ppm C₃H₆-3000 ppm O₂—3.5% CO—balance being N₂gas at a temperature elevation rate of 10° C./min from 250 to 750° C. atan sulfur component trapping agent entrance, after the trapping test. 3.An exhaust gas purification apparatus for an internal combustion engine,which comprises an exhaust gas passage for an internal combustion engineinto which exhaust gas of lean air fuel ratio and rich or stoichiometricair fuel ratio flows, a NOx trapping catalyst that functions to trap NOxin the exhaust gas when the air fuel ratio is lean, a sulfur componenttrapping agent for trapping sulfur components in the exhaust gas, whichis disposed before the NOx trapping catalyst, and a catalyst foroxidizing the sulfur components, which is disposed before the sulfurcomponent trapping agent, wherein the sulfur component trapping agenthas a trapping rate of 60% or more of an amount of inflow sulfur in atrapping test at a flow rate of 150 ppm H₂S—0.5% O₂—balance being N₂ gasat 300° C. of the sulfur trapping agent and a space velocity of 30,000/hfor 1 hour.
 4. An exhaust gas purification apparatus for an internalcombustion engine, which comprises a NOx trapping catalyst for trappingNOx, which is disposed in an exhaust gas passage, a sulfur componenttrapping agent disposed before the NOx trapping catalyst for trappingsulfur components, and a catalyst disposed before the sulfur componenttrapping agent for oxidizing the sulfur components, wherein the sulfurcomponent trapping agent contains at least one of alkali metals andalkaline earth metals and a total amount of Pt, Pd and Rh is at least0.4% by weight of the sulfur component trapping agent.
 5. The exhaustgas purification apparatus according to claim 1, wherein the sulfatescontained in the sulfur component trapping agent has a meltingtemperature or decomposition temperature of 750° C. or higher.
 6. Theexhaust gas purification apparatus according to to claim 1, wherein thesulfur component trapping agent is disposed below the engine.
 7. Theexhaust gas purification apparatus according to claim 1, which furthercomprises a filter disposed in upstream of the NOx trapping catalyst,wherein an upstream side of the filter is provided with a catalyst foroxidizing the sulfur components and a downstream side of the filter isprovided with the sulfur component trapping agent.
 8. The exhaust gaspurification apparatus according to claim 1, which further comprises afilter disposed at upstream of the NOx trapping catalyst, wherein thesulfur component trapping agent if formed on part of the filter, and thecatalyst for oxidizing sulfur components is formed on another part ofthe filter.
 9. The exhaust gas purification apparatus according to claim4, wherein an amount of the alkali metals or the alkaline earth metalsis 1 to 4 moles or less in terms of (alkali metals /2+alkaline earthmetals).
 10. The exhaust gas purification apparatus according to claim1, wherein the catalyst for oxidizing sulfur components contains atleast one of Pt, Pd and Rh.
 11. The exhaust gas purification apparatusaccording to claim 1, wherein the NOx trapping catalyst contains atleast one of alkali metals and alkaline earth metals and at least one ofnoble metals, and has a function to trap SOx under a lean air fuelcondition and a function to release SOx in a rich or stoichiometric airfuel condition by heating the catalyst to 500° C. or higher.
 12. Theexhaust gas purification apparatus according to claim 1, wherein thesulfur component trapping agent is replaceable with another.
 13. Asulfur component trapping agent containing an ingredient for trappingsulfur components in an exhaust gas, wherein the ingredient has atrapping rate of 85% or more of an amount of inflow sulfur in a trappingtest at a flow rate of 150 ppm SO₃-5% O₂—balance being N₂ gas per 1.5moles of the sulfur trapping agent at 300° C. and a space velocity of30,000/h for 1 hour; and the sulfur component trapping agent has arelease rate of sulfur amount of 5% or less of sulfur trapped in thesulfur component trapping agent in a release test under a flow of a 3000ppm H₂—600 ppm C₃H₆—3000 ppm O₂—3.5% CO—balance being N₂ gas at atemperature elevation rate of 10° C./min from 250 to 750° C. at ansulfur component trapping agent entrance, after the trapping test. 14.The sulfur component trapping agent according to claim 13, wherein thesulfur component trapping agent contains at least one selected from thegroup consisting of alkali metals, alkaline earth metals, Ce, Al, Y, Laand Ni.
 15. A sulfur component trapping agent for trapping sulfur in anexhaust gas, which comprises a honeycomb substrate made of cordierite ormetal, a porous support, and a sulfur trapping agent supported on theporous support, wherein the sulfur trapping agent contains 1 to 4 molesof at least one of alkali metals and alkaline earth metals in (molarnumber of alkali metals /2+molar number of alkaline earth metals) asconversion of elements, and the total amount of Pt+Pd+Rh is 0.4% byweight or more per the sulfur component agent.
 16. A method ofpurification of an exhaust gas from an internal combustion engine, whichuses the sulfur component trapping agent according claim
 13. 17. Amethod of purification of an exhaust gas for an internal combustionengine, which comprises oxidizing sulfur components in the exhaust gas,trapping and accumulating the sulfur components in a sulfur componenttrapping agent, and purifying NOx in the exhaust gas with a NOxpurifying catalyst.
 18. The method of purification of an exhaust gasaccording to claim 16, which comprises a step for releasing the sulfurcomponents from the NOx purifying catalyst, wherein the releasing stepis carried out by changing the air fuel ratio to rich or stoichiometricand elevating temperature of the NOx purifying catalyst to 500° C. orhigher.
 19. A method of diagnosis of degradation of a sulfur componenttrapping agent in an exhaust gas purification apparatus comprising a NOxpurification catalyst, a sulfur component trapping agent disposed beforethe NOx purification catalyst, and a sulfur component oxidizing catalystdisposed before the sulfur component trapping agent, which comprisesmeasuring NOx purification rates before and after a step of releasing asulfur component from the NOx purification catalyst and diagnosing adegradation of the sulfur component trapping agent based on a differenceor ratio of the NOx purification rates.
 20. A system for diagnosis ofdegradation of a sulfur component agent in an exhaust gas purificationapparatus comprising a NOx purification catalyst for trapping NOx, asulfur component trapping agent disposed before the NOx purificationcatalyst for trapping sulfur components, a sulfur component oxidizingcatalyst disposed before the sulfur component trapping agent, whichcomprises means for diagnosing the sulfur component trapping agent inaccordance with the diagnosis method defined in claim 19 for everysulfur component releasing step, and means for indicating replacement ofthe sulfur component trapping agent when the sulfur component trappingagent is degraded to a predetermined level.